In an effort to provide sufficient channel bandwidth to meet expected subscriber demand in a public ISDN environment, the art has turned to implementing so-called broadband ISDN (B-ISDN). In B-ISDN, each subscriber channel is presently envisioned as providing an information transfer capacity of approximately 150 Mbit/S. This rate is chosen to provide a minimally sufficient bandwidth at a subscriber interface to simultaneously carry a broadband video service, such as high definition television (HDTV) and various narrowband services, such as voice and data transmission. Packet switched connections, instead of circuit switched connections, specifically utilizing asynchronous transfer mode (ATM), is the preferred mode of communications over B-ISDN.
Today, large central offices could have up to 100,000 customers. A future broadband ISDN (B-ISDN) central office will be likely to require a switching capacity of 1 Terabit/S (10.sup.12 bit/S) or more. It is estimated that in B-ISDN, each customer will be served with an STS-3c line (operating at 155.52 Mbit/S). Assuming a utilization factor of about 10%, an associated asynchronous transfer mode (ATM) switch will have to handle the capacity of about 10,000 STS-3c lines, or about 1.5 terabit/sec.
Several proposals have been made, within a factor of 10 of a terabit/sec in capacity, for the architecture of such large ATM switches. Almost all use electronic switching components exclusively. An exception, which uses an optical star coupler, is the STAR-TRACK optical multicast switch. The STAR-TRACK optical multicast switch is described in U.S. Pat. Nos. 4,896,934 and 5,005,167, both issued to Arthurs et al. and assigned to the assignee of the present invention. Operation of this switch includes two control phases and a transmission phase. During the control phases, a token is generated and passed sequentially along a track from one input port to another. Each input port writes information into the token indicating the output ports to which their packets are to be sent. The token is then passed sequentially along the track from one output port to the next. The output ports read the token and tune their receivers to the appropriate input port wavelength. During the transmission phase, the packets are then optically transmitted from the input ports to the output ports. Thus, contention resolution involves token passing, a process which not only limits the number of ports capable of being serviced, but which slows the switch capacity.
The possibility of purely photonic packet switching is at present severely limited by the difficulty of buffering packets and reading packet headers in the photonic domain. The buffering function is necessary in a packet switch unless it is unrealistically overbuilt. At present, this function can only be provided electronically in the needed capacity.